objective-c - "unrecognized selector"尝试访问 CIFilter 的 outputImage

Question

我第一次尝试使用 Core Image(在 OS X，10.7.3 上)，但遇到了障碍。我确定这是我在做的愚蠢的事情，只需要更熟悉该框架的人向我指出。

考虑以下代码(让我们规定 imageURL 是指向磁盘上 JPG 的有效文件 URL):

CIImage *inputImage = [CIImage imageWithContentsOfURL:imageURL];
CIFilter *filter = [CIFilter filterWithName:@"CIAreaAverage" keysAndValues:
                                                  kCIInputImageKey, inputImage,
                                                  kCIInputExtentKey, [inputImage valueForKey:@"extent"],
                                                  nil];
CIImage *outputImage = (CIImage *)[filter valueForKey:@"outputImage"];

运行这段代码时，最后一行触发:

0   CoreFoundation                      0x00007fff96c2efc6 __exceptionPreprocess + 198
1   libobjc.A.dylib                     0x00007fff9153cd5e objc_exception_throw + 43
2   CoreFoundation                      0x00007fff96cbb2ae -[NSObject doesNotRecognizeSelector:] + 190
3   CoreFoundation                      0x00007fff96c1be73 ___forwarding___ + 371
4   CoreFoundation                      0x00007fff96c1bc88 _CF_forwarding_prep_0 + 232
5   CoreImage                           0x00007fff8f03c38d -[CIAreaAverage outputImage] + 52
6   Foundation                          0x00007fff991d8384 _NSGetUsingKeyValueGetter + 62
7   Foundation                          0x00007fff991d8339 -[NSObject(NSKeyValueCoding) valueForKey:] + 392

现在，Core Image Filter Reference 明确指出 CIAreaAverage “返回包含感兴趣区域平均颜色的单像素图​​像。”事实上，更令人困惑的是，当我在调试器中检查过滤器属性时(在尝试调用 valueForKey: 之前):

(lldb) po [filter attributes]
(id) $3 = 0x00007fb3e3ef0e00 {
    CIAttributeDescription = "Calculates the average color for the specified area in an image, returning the result in a pixel.";
    CIAttributeFilterCategories =     (
        CICategoryReduction,
        CICategoryVideo,
        CICategoryStillImage,
        CICategoryBuiltIn
    );
    CIAttributeFilterDisplayName = "Area Average";
    CIAttributeFilterName = CIAreaAverage;
    CIAttributeReferenceDocumentation = "http://developer.apple.com/cgi-bin/apple_ref.cgi?apple_ref=//apple_ref/doc/filter/ci/CIAreaAverage";
    inputExtent =     {
        CIAttributeClass = CIVector;
        CIAttributeDefault = "[0 0 640 80]";
        CIAttributeDescription = "A rectangle that specifies the subregion of the image that you want to process.";
        CIAttributeDisplayName = Extent;
        CIAttributeType = CIAttributeTypeRectangle;
        CIUIParameterSet = CIUISetBasic;
    };
    inputImage =     {
        CIAttributeClass = CIImage;
        CIAttributeDescription = "The image to process.";
        CIAttributeDisplayName = Image;
        CIUIParameterSet = CIUISetBasic;
    };
    outputImage =     {
        CIAttributeClass = CIImage;
    };
}

那里有 outputImage - 以 CIImage 类型给出!

那么，我做错了什么？我看到的所有文档和教程都表明 -valueForKey: 是访问属性的正确方法，包括 outputImage。

Answer 1

我相信您的范围是罪魁祸首(无论它多么奇怪)。当我将范围更改为 CIVector* 时，它起作用了。

NSURL *imageURL = [NSURL fileURLWithPath:@"/Users/david/Desktop/video.png"];
CIImage *inputImage = [CIImage imageWithContentsOfURL:imageURL];
CIFilter *filter = [CIFilter filterWithName:@"CIAreaAverage"];
[filter setValue:inputImage forKey:kCIInputImageKey];
CGRect inputExtent = [inputImage extent];
CIVector *extent = [CIVector vectorWithX:inputExtent.origin.x
                                       Y:inputExtent.origin.y
                                       Z:inputExtent.size.width
                                       W:inputExtent.size.height];
[filter setValue:extent forKey:kCIInputExtentKey];
CIImage *outputImage = [filter valueForKey:@"outputImage"];

[inputImage extent] 返回一个 CGRect，但显然 CIVector* 效果更好。